Semiconductor memory device

ABSTRACT

A static type semiconductor RAM device having a plurality of memory cells disposed at the cross points between word lines and bit lines, load circuits each connected between one of the bit lines and a voltage source, and, a charging circuit which electrically charges all the bit lines during the short time when all the word lines are in a non-selected condition.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a semiconductor memory device, more particularly to a static type semiconductor RAM (Random Access Memory) device having a large memory capacity and a high operating speed.

2. Description of the Prior Art

In general, the current consumption of memory cells and bit lines of a static type RAM device becomes large according to an increase in the memory capacity, i.e., according to an increase in the number of the memory cells. Therefore, especially in a RAM device having a large memory capacity, the gm of each of the load transistors connected to the bit lines is caused to be small in order to decrease the current consumption of the bit lines.

As illustrated in FIG. 1, a conventional static type RAM device comprises a plurality of memory cells MC₀,0, MC₁,0, . . . ; word lines W₀, W₁, . . . ; pairs of bit lines B₀ and B₀, . . . ; word decoders WD₀, WD₁, . . . ; and load MIS transistors Q₁, Q₂, . . . for each of the bit lines B₀ and B₀. Each of the memory cells MC₀,0, MC₁,0, . . . has the same circuit structure and comprises load resistors R₁ and R₂ and MIS transistors Q₃ through Q₆. The drain electrodes and the gate electrodes of the transistors Q₅ and Q₆ are cross-coupled so as to form a flip-flop type memory cell circuit.

In the memory device of FIG. 1, when, for example, the word line W₀ is selected by address signals ADD and the output potential of the word decoder WD₀, i.e., the word line W₀ becomes the potential level for a selected condition, for example high, the transistors Q₃ and Q₄ are both turned on. In this condition, if the transistors Q₆ of the memory cell MC₀,0 is in an on state, a current flows from a voltage source V_(DD) through the transistors Q₂, Q₄ and Q₆ to another voltage source V_(SS), so that the potential of the bit line B₀ becomes low. Since the transistor Q₅ of the same memory cell MC₀,0 is in a turned off condition, the potential level of the bit line B₀ is high. The potential difference between the bit lines B₀ and B₀ is detected by a sense amplifier (not shown in the drawing) and information from the selected memory cell MC₀,0 is provided as an output.

In the above-mentioned RAM device, if the memory cell MC₁,0 is selected next after the memory cell MC₀,0 and if the memory cell MC₁,0 stores a different data from that of the memory cell MC₀,0, i.e., a transistor Q'₅ is in a turned on condition and a transistor Q'₆ is in a turned off condition, it is necessary to electrically charge the bit line B₀ to a high potential level quickly. However, since, as aforementioned, the gm of the load transistors Q₁ and Q₂ is caused to be small in order to decrease the current consumption of the bit lines of a memory device having a large memory capacity, it is impossible to quickly charge the bit line B₀ from low to high. Therefore, the access time of such a conventional memory device cannot be short. If the read-out of information from the memory cell MC₁,0 is effected just after the write-in of information to the memory cell MC₀,0 and if the memory cell MC₁,0 stores data different from that written into the memory cell MC₀,0, it is necessary to electrically charge the bit line, for example, B₀, from a very low potential (e.g. 0 V) to a high potential (e.g. 3 V), so that the access time of the RAM device becomes still longer than that in the above-mentioned case.

SUMMARY OF THE INVENTION

It is, therefore, the principal object of the present invention to decrease the access time of a static type RAM device having a large memory capacity without increasing the current consumption in a steady state operation.

According to the present invention, there is provided a semiconductor memory device comprising a plurality of word lines, a plurality of bit lines, a plurality of memory cells disposed at the cross points between the word lines and the bit lines and load circuits each connected between one of the bit lines and a voltage source, a memory cell disposed at the cross point of a word line and a bit line being selected by selecting said word line and the bit line, characterized in that the memory device comprises, besides the load circuits, a charging circuit which electrically charges all the bit lines during the switching period of word address signals.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a partial circuit diagram illustrating a known conventional static type RAM device;

FIG. 2 is a partial circuit diagram illustrating a static type RAM device according to the present invention; and

FIG. 3A through FIG. 3E are waveform diagrams illustrating the operation of the RAM device of FIG. 2.

DESCRIPTION OF THE PREFERRED EMBODIMENT

FIG. 2 illustrates a static type RAM device as an embodiment of the present invention. In FIG. 2, the same parts as appear in FIG. 1 are designated by the same reference symbols. The RAM device of FIG. 2 includes a plurality of memory cells MC₀,0, . . . , MC₀,M-1, MC₁,0, . . . , MC₁,M-1, . . . disposed at cross points between a plurality of word lines W₀, W₁, . . . and a plurality of pairs of bit lines B₀ and B₀ through B_(M-1) and B_(M-1) ; a plurality of word decoders WD₀, WD₁, . . . connected to the word lines W₀, W₁, . . . respectively; and pairs of gate transistors Q₂₀₀ and Q₂₁₀ through Q₂₀(M-1) and Q₂₁(M-1) connected to the pairs of bit lines B₀ and B₀ through B_(M-1) and B_(M-1) respectively, and controlled by column decode signals Y₀ through Y_(M-1) respectively. The RAM device of FIG. 2 further comprises load transistors Q₁₀ and Q₂₀ through Q₁(M-1) and Q₂(M-1) connected between the pairs of the bit lines B₀ and B₀ through B_(M-1) and B_(M-1) and a power source V_(DD) respectively; a write-in circuit WRC constituted by an amplifier WA and three NAND gates NG1 through NG3, whose output terminals are connected to a pair of data bit lines DB and DB; a sense amplifier SA connected to the pair of data bit lines DB and DB; and an output buffer amplifier BA connected to the sense amplifier SA.

The RAM device of FIG. 2 still further comprises, according to the present invention, a high speed charging circuit constituted by MIS charging transistors Q₁₁₀ and Q₁₂₀ through Q₁₁(M-1) and Q₁₂(M-1) and by MIS transistors Q₁₃, Q₁₄, Q₁₅, Q₁₆, Q₁₇, . . . . The MIS charging transistors Q₁₁₀ and Q₁₂₀ through Q₁₁(M-1) and Q₁₂(M-1) are connected parallel to the load transistors Q₁₀ and Q₂₀ through Q₁(M-1) and Q₂(M-1), respectively. The MIS transistors Q₁₃, Q₁₄, Q₁₅, Q₁₆, Q₁₇, . . . constitute a NOR gate to generate a control signal S_(C) supplied to the gate electrodes of the MIS charging transistors Q₁₁₀ and Q₁₂₀ through Q₁₁(M-1) and Q₁₂(M-1). The MIS transistor Q₁₃ is a load transistor of the NOR gate. The gate electrodes of the MIS transistors Q₁₄ and Q₁₅ are connected to the word line W₀ at both ends thereof and the gate electrodes of the MIS transistors Q₁₆ and Q₁₇ are connected to the word line W₁ at both ends thereof. The other MIS transistors connected to the other word lines at both ends thereof are not shown in the drawing.

The charging transistors Q₁₁₀ and Q₁₂₀ through Q₁₁(M-1) and Q₁₂(M-1) are controlled by the output signal, i.e., the control signal S_(C). That is, when all the transistors Q₁₄, Q₁₅, Q₁₆, Q₁₇, . . . are in a turned off condition, the output signal S_(C) of the NOR gate becomes high so that all the charging transistors Q₁₁₀ and Q₁₂₀ through Q₁₁(M-1) and Q₁₂(M-1) are turned on. On the contrary, when at least one of the transistors Q₁₄, Q₁₅, Q₁₆, Q₁₇, . . . is in a turned on condition, the output signal S_(C) of the NOR gate becomes low so that all the charging transistors Q₁₁₀ and Q₁₂₀ through Q₁₁(M-1) and Q₁₂(M-1) are turned off. The transistors Q₁₄ and Q₁₅ (or Q₁₆ and Q₁₇ or other transistors not shown in the drawing) detect the potential levels of one end portion of the word line W.sub. 0 (or W₁, . . . ) and of another end portion of the same word line W'₀ (W'₁, . . . ). Therefore, the charging transistors Q₁₁₀ and Q₁₂₀ through Q₁₁(M-1) and Q₁₂(M-1) are not turned on until the potential of all portions of the word lines W₀, W₁, . . . becomes low level, i.e., the level of the non-selected conditions even when the potential of each portion of the word line does not change at the same time due to the propagation delay of the word line along the longitudinal direction thereof. On the contrary, the charging transistors Q₁₁₀ and Q₁₂₀ through Q₁₁(M-1) and Q₁₂(M-1) are turned on immediately after the output potential of at least one of the word decoders WD₀, WD₁, . . . becomes high level, i.e., the potential level in the selected condition of the word line.

With reference to FIG. 3A through FIG. 3E, operation of the above-mentioned RAM device will now be explained. Assume that, as illustrated in FIG. 3A, the word address signal ADD has changed from ADD(W₀) which selects the word line W₀ to ADD(W₁) which selects the word line W₁ at a time t₀ and pairs of bit lines B₀ and B₀ are continuously selected before and after the time t₀. That is, the memory cell MC₀,0 is selected before the time t₀ and the memory cell MC₁,0 is selected after the time t₀. Assume also that the read-out operations from both the memory cells MC₀,0 and MC₁,0 are sequentially effected and the memory cells MC₀,0 and MC₁,0 store different information from each other. In this condition, if the transistor Q₅ of the memory cell MC₀,0 is in a turned off condition and the transistor Q₆ of the same memory cell is in a turned on condition, the potentials of the bit lines B₀ and B₀ are high (e.g. 3 V) and low (e.g. 2 V) respectively (FIG. 3D), the potentials of the word lines W₀ and W₁ are high and low respectively and the potentials of the end portions W'₀ and W'₁ of the word lines W₀ and W₁ are also high and low, respectively (FIG. 3B), at the time t₀. After the word address signal ADD has changed from ADD(W₀) to ADD(W₁), the output signal of the word decoder WD₀, i.e., the potential of the word line W₀, changes from high to low and the output signal of the word decoder WD₁, i.e., the potential of the word line W₀, changes from low to high. In this case, the potentials of the end portions W'₀ and W'₁ of the word lines W₀ and W₁ change a short time after the potential of the beginning portions of the word lines W₀ and W₁ have changed. In FIG. 3B, the curves of the potentials of the beginning portion of the word lines W₀ and W₁ are designated by the symbols W₀ and W₁ respectively. As a result, all the input signals of the NOR gate constituted by the translstors Q₁₃ through Q₁₇ and others become low at a time t₁ when the potential of the end portion W'₀ of the word line W₀ becomes low, so that the output signal S_(C) of the NOR gate becomes high as illustrated in FIG. 3C. At a time t₂, the output potential W₁ of the word decoder WD₁ becomes high and the output signal S_(C) of the NOR gate becomes low. Therefore, during the time period between t₁ and t₂, no word line is selected and the output signal S_(C) becomes high. In the present invention, the charging transistors Q₁₁₀ and Q₁₂₀ through Q₁₁(M-1) and Q₁₂(M-1) are turned on during this non-selected time period and the bit lines B₀ and B₀ are quickly charged through the charging transistors Q₁₁₀ and Q₁₂₀ and through the load transistors Q₁₀ and Q₂₀. Therefore, the potential curve of the bit line B₀ rises quickly and crosses the potential curve of the bit line B₀ at a time t₃ as illustrated in FIG. 3D. A curve B'₀ shown by the dotted line of FIG. 3D illustrates the charging characteristics of the conventional memory device of FIG. 1, in which the bit lines B₀ and B₀ are charged only by the load transistors Q₁ and Q₂, respectively. The dotted line curve B'₀ crosses the curve B₀ at a time t₄ which is later than the time t₃ by Δt. Since the transfer gate transistors Q₂₀₀ and Q₂₁₀ of FIG. 2 connected between the bit lines B₀ and B₀ and the data bit lines DB and DB are in a turned on condition, the abovementioned potential changes are transferred to the sense amplifier SA and the buffer amplifier BA. The sense amplifier detects the potential change of the bit lines B₀ and B₀ and changes the potential of the read-out signal D_(out) after the cross point of the potential curves B₀ and B₀ (or B'₀). Therefore, according to the present invention, the access time of the RAM device can be increased by Δt. Since the chargng transistors Q₁₁₀ and Q₁₂₀ through Q₁₁(M-1) and Q₁₂(M-1) are only turned on temporarily, the current consumption of the RAM device does not increase much.

In the above-mentioned memory device, the write-in of information to a memory cell, for example, MC₀,0 is effected by applying a write-in data signal D_(in) and an inverted write enable signal WE of low potential level to the write-in circuit WRC and by turning on the transfer gate transistors Q₂₀₀ and Q₂₁₀ by the column decode signal Y₀ and the transistors Q₃ and Q₄ by the output signal of the word decoder WD₀. Assume that the potentials of the data bit lines DB and DB are caused to be high (e.g. 3 V) and low (e.g. 0 V) respectively according to the write-in data signal D_(in) supplied to the write-in circuit WRC. These high and low potentials are transferred to the memory cell MC₀,0 through the transfer gate transistors Q₂₀₀ and Q₂₁₀ and through the transistors Q₃ and Q₄ and the write-in of information is effected.

If the read-out operation from the memory cell MC₁,0 is effected after the above-mentioned write-in to the memory cell MC₀,0 is effected and, if the data stored in the memory cell MC₁,0 is different from the data written into the memory cell MC₀,0, the bit line B₀ must be charged from the low potential (0 V) of the write-in mode to the high potential (3 V) of the read-out mode quickly. In the RAM device according to the present invention, the charging transistors Q₁₁₀ and Q₁₂₀ through Q₁₁(M-1) and Q₁₂(M-1) are also turned on temporarily in a similar manner to the aforementioned case, and, therefore, the potential of the bit line B₀ rises quicker than that of the conventional RAM device as illustrated in FIG. 3E. Therefore, in even the worst case, the access time of the RAM device of the present invention can be much shorter than that of the conventional RAM device.

As mentioned above, since the RAM device according to a present invention comprises the high speed charging circuit which charges all the bit lines during a short time when all the word lines are in a non-selected condition, the access time of the RAM device having a large memory capacity can be short without increasing the power consumption much, even when the size, and therefore the gm, of each of the load transistors of the bit lines is small. 

I claim:
 1. An improved semiconductor memory device of the type including a plurality of word lines, a plurality of bit lines crossing said word lines, a plurality of memory cells disposed at the cross points between said word lines and said bit lines, and load circuits each connected between one of said bit lines and a voltage source, a memory cell disposed at the cross point of a word line and a bit line being selected by selecting said word line and said bit line, wherein the improved memory device further comprises charging circuit means, operatively connected in parallel with said load circuits, for detecting the switching period when the potentials of all the word lines are at the potential level of a non-selected condition and for electrically charging all the bit lines during the switching period of word line potentials.
 2. The memory device of claim 1, wherein said charging circuit means comprises gate circuit means for detecting the potential levels of all of said word lines and a plurality of charging transistors which are controlled by the output signal from said gate circuit means.
 3. The memory device of claim 2, wherein said gate circuit means comprises a NOR gate having a plurality of input transistors and a load transistor connected to said input transistors.
 4. The memory device of claim 2, wherein each of said charging transistors is connected parallel to a corresponding one of said load circuits.
 5. The memory device of claim 2, wherein each of said word lines has a beginning portion and an end portion, and wherein said gate circuit means comprises means for detecting the potential levels at the beginning portion and the end portion of each of said word lines.
 6. The memory device of claim 5, wherein said gate circuit means comprises a NOR gate having a plurality of input transistors numbering double the number of said word lines, and a load transistor, each of said input transistors being connected between one of the beginning portion of a word line and said load transistor and between the end portion of a word line and said load transistor.
 7. A memory device of claim 1, 2, 3, 4, 5, or 6, wherein each of said memory cells is a flip-flop type memory cell comprising a pair of cross coupled MIS transistors, a pair of transfer gate MIS transistors and a pair of load resistors. 